In today's telecommunications industry, market forces are driving communication devices to support enhanced data rates as well as more efficient power consumption to conserve battery life. Much of the power in a communication device is consumed to support the power amplifier and associated circuits. Even when power ranges vary, power amplifiers should still operate at peak efficiency. In an effort to maximize efficiency, power amplifiers must operate near peak efficiency over the varying power ranges that can be called for in a communication device.
Where the power amplifier is incorporated in a system with a polar modulation transmitter, the demands on the power amplifier can be higher in comparison to demands on a Cartesian system, for example. In a Cartesian system, a modulated signal is amplified by a power amplifier having a power control signal for controlling the average output power to the required power level. In a polar modulation system, in addition to the usual power level control requirements, the dynamic range requirement is more demanding because the power amplifier must support an adequate dynamic range, from minimum value to maximum values. In addition, the polar system should support the modulation envelope appropriate for the transmission.
To consider a specific example, the modulation envelope for enhanced data rate for GSM system evolution (EDGE) systems requires about 16–17 dB range. In addition to the modulation envelope, EDGE requires about 30 DB of power control, for a total dynamic range of approximately 50 dB. Accordingly, the polar modulation system needs to operate at about 50 dB dynamic range if designed to work with EDGE.
Where a conventional power amplifier is utilized with a polar modulation transmitter, the power control signal magnitude is very low, i.e., near zero, at low output power levels. (This of course assumes that the power amplifier has sufficient dynamic range in the first place.) The low power control signal magnitude can exacerbate issues caused by DC offsets in the amplitude path and carrier leakage in the power amplifier, leading to high levels of signal distortion. Moreover, the conventional power amplifier does not operate as efficiently as possible at high output power, e.g., in EDGE mode, if an alternate higher output power mode, such as a GSM constant envelope mode is supported. Further, the conventional power amplifier does not operate efficiently at reduced output power levels.